Marina Litvinenko in 2000 with her husband, Alexander, a former Russian agent who died from radiation poisoning and whose inquest is the subject of controversy over withheld of government papers. Photograph: Francesco Guidicini/Rex Features

If you want to know why you should not support the coalition's plans for secret courts, consider the British state's treatment of Marina Litvinenko.

Before Christmas, she was as happy as a woman in her position could be. Her husband, Alexander, had helped MI6 and the Spanish secret service deal with the flood of mafia money into western Europe. Litvinenko was a former agent of Russia's Federal Security Service and his past left him well placed to give informed advice. It also made him a marked man. He went to a London hotel in the autumn of 2006 and met two Russians, whom the British authorities later identified as Dmitry Kovtun and Andrei Lugovoi. He fell ill hours later. Doctors found polonium-210 in his bloodstream. His killers had administered an exemplary punishment. Anyone else who thought of crossing Russia would think again after reading of the agonising 23 days Litvinenko spent on his deathbed.

After detectives visited him, his wife reported: "Those were tough policemen but they were almost crying when they came out of his room."

Russia refused to extradite Kovtun and Lugovoi. The police investigation had nowhere to go, so the inquest could begin this year. Marina Litvinenko was relieved. "I want this inquest to show the truth about what happened and I want to protect my husband's name from all the lies that have been told about him in Russia," she said.

It is hard to avoid a descent into psychobabble in such situations and I won't try. I know from when my dear father disappeared in the Lake District that families want "closure". Only an explanation of the circumstances of a suspicious death allows you to lay the body of your loved one down, to let him rest in peace. Marina Litvinenko thought that Britain would grant her that consolation. As the late law lord Thomas Bingham explained, "for centuries" suspicious deaths in this country have been "publicly investigated before an independent judicial tribunal with an opportunity for relatives of the deceased to participate".

The reasons Bingham gave for open justice apply in the Litvinenko case more than any other I can think of. "To ensure so far as possible that the full facts are brought to light" – and a political murder cries out for explanation. "That culpable and discreditable conduct is exposed" – and what else is poisoning but culpable and discreditable? "That suspicion of deliberate wrongdoing (if unjustified) is allayed" – and, as Mrs Litvinenko says, she needs a competent court to answer the conspiracy theorists. Finally, Bingham said that secrecy must be resisted so "that those who have lost their relative may at least have the satisfaction of knowing that lessons learned from his death may save the lives of others" – and I do not need to explain the relevance of those words to the Litvinenko family.

William Hague is showing his contempt for the best British traditions by seeking to stop the inquest examining government papers on the murder. Openness would cause "serious harm to the national security and/or international relations," the Foreign Office says. I love its "and/or". This is Whitehall's equivalent of Groucho Marx's "Those are my principles, and if you don't like them… well, I have others." Without the wit and with considerably more menace, government lawyers say that if the court doesn't believe that telling the truth will harm national security, will it believe it will harm international relations instead?

Hague's demand for secrecy looks like bureaucratic extremism, the product of a constipated government machine that never wants to let information out. How can Hague and MI6 defend it? They cannot claim they want to protect a secret agent from harm – Alexander Litvinenko is dead and in his grave. No one can harm him there. Maybe they want to deny that he worked for MI6. But his wife has bank records of payments from shell companies and knew his minder. Meanwhile, Hugh Davies, counsel to the inquiry, said in open court that the government had evidence that the hit on Litvinenko was authorised by the Russian state, so that is no secret either.

One other bitter truth is incontestable: the government is treating Mrs Litvinenko in a shameful manner. All kinds of powerful organisations are hanging round Sir Robert Owen's coroner's court. Not only the Foreign Office, media and Ministry of Defence, but also the Investigative Committee of the Russian Federation, sometimes described as Russia's FBI. All can hire the best lawyers. The only person who cannot is Mrs Litvinenko.

Chris Grayling's Justice Department has refused her legal aid. It wants to deny the widow of the victim of one of the most sensational murders in recent history proper legal representation. So she is relying on charity. Ben Emmerson QC has agreed to represent her for nothing and makes little effort to conceal his disdain for Whitehall. David Cameron said last year that he was keen to work with the Russian government to strengthen business links. Emerson alleges that for the sake of trade with a country that habitually fleeces foreign investors, Cameron is prepared to hush up allegations that its agents poisoned one of Putin's enemies in London.

The government could prove him wrong by allowing a public inquiry into the Litvinenko affair. It shows no sign of wanting that and, ominously, the coroner has just announced that he will examine evidence in secret before deciding whether open justice can take its course.

Civil libertarians misquote Benjamin Franklin's injunction to "sell not virtue to purchase wealth, nor Liberty to purchase power" and have turned it into "if we restrict liberty to attain security we will lose them both". This is not what Franklin said and is not true either. David Anderson QC, the independent reviewer of terrorism legislation and a man you should always take seriously, believes there is a "small but indeterminate category of national security-related claims" in which closed hearings would be justified. I trust him on that.

The trouble is that you cannot trust the state to confine itself to a "small but indeterminate" number of cases. When Parliament passed the Regulation of Investigatory Powers Act in 2000, ministers promised they would only use the new surveillance techniques against terrorists and gangsters. Local authorities ended up using them against fly-tippers.

Now the state wants secret courts and promises it will only lock their doors in exceptional circumstances. You need only look at the cheated and bewildered figure of Marina Litvinenko to know it is lying.• This article will be opened for comments on Sunday morning